Method for the optical analysis of a fluid

ABSTRACT

The invention concerns a method for the optical analysis of a fluid, in which the fluid that has a first refractive index, is surrounded by an auxiliary fluid with a second refractive index, the first refractive index being larger than the second refractive index and light being sent through the fluid. It is endeavoured to obtain an encapsulation of the fluid in the auxiliary fluid at low cost. For this purpose, firstly the auxiliary fluid is led into a measuring channel, until it is filled, and then lead the fluid into the auxiliary fluid within the cross section with a flow speed, which generates a parabolic flow profile.

[0001] Method for the optical analysis of a fluid The invention concernsa method for the optical analysis of a fluid, in which the fluid thathas a first refractive index, is surrounded by an auxiliary fluid with asecond refractive index, the first refractive index being larger thanthe second refractive index and light being sent through the fluid.

[0002] Such a method is known from DE 195 36 858 A1. Such methods areparticularly suited for light spectroscopic processes, in which thelight is led through the fluid with a predetermined wave length or apredetermined wave length range. It can then be measured, how good thelight gets through the fluid (transmission) or which spectral shares areabsorbed (absorption). From these results, conclusions concerningcertain substances can be made, the existence of which in the fluid isto be proved.

[0003] Encapsulating the fluid in an auxiliary fluid involves theadvantage, among others, that the measuring channel is not impurified.Additionally, the optical properties of the measuring channel wall, forexample, the reflection properties, are less important, so that cheapermaterials can be used for the manufacturing of the measuring channel.The refractive indexes of fluid and auxiliary fluid can be adapted toeach other in such a way that the introduced light is reflected at theboundary layer between fluid and auxiliary fluid, to remain in the fluidfor as long as possible. With a suitable adaptation, a fluid opticalconductor can be obtained, which has a total internal reflection. When atotal internal reflection is available, the light is not merelyreflected, but also literally led through the conductor.

[0004] In the method known from DE 195 36 858 A1, the fluid and the corefluid are introduced simultaneously in a laminating device, in which thefluids are laminated in relation to each other, though maintaining thesame direction and the same speed. Thus, it is, in a manner of speaking,an active encapsulating process. This sort of encapsulation requires arelatively expensive device. Additionally, also the pumps supplying thefluids must meet high requirements, particularly with regard to thesupply quantity accuracy.

[0005] The invention is based on the task of performing theencapsulation of the fluid in an auxiliary fluid at low cost.

[0006] With a method as mentioned in the introduction, this task issolved in that firstly the auxiliary fluid is led into a measuringchannel, until it is filled, and secondly the fluid is led into theauxiliary fluid within the cross section with a flow speed, whichresults in a parabolic flow profile.

[0007] In this connection, it is not necessary that the fluid issupplied exactly in the centre of the cross section of the auxiliaryfluid. It is sufficient that the supply occurs in the middle of thecross section. In this way, lamination of the two fluids is relativelyeasily realised. The fluid flowing in displaces the auxiliary fluid andflows through the auxiliary fluid in such a way that a layer of theauxiliary fluid, surrounding the fluid, remains between the wall of themeasuring channel and the fluid. In this connection, the speed of thefluid is set so that a parabolic flow profile occurs. This flow profileoccurs with a laminar flow, so that between the fluid and the auxiliaryfluid a turbulence does not occur, but merely a boundary layer iscreated, on which the supplied light can be reflected due to thedifference in the refractive indexes. The parabolic flow profile forms apeak that, being cone-shaped, pushes the auxiliary fluid away andpenetrates it. The exact speed of the fluid depends on severalparameters, for example on the density difference between the twofluids. Smaller cross sections or longer measuring channels requirehigher flow speeds. However, depending on the physical dimensions andthe fluids chosen,-there is an upper limit on the flow speed, at which aturbulence appears and the laminar structure collapses. However, aperson skilled in the art can easily find the required flow speed byperforming a few tests. An additional advantage of the invention is thatthe requirements on the pump, transporting the fluid through themeasuring channel, are less critical. In the known case, in which thefluids had to be laminated to each other with the same speed, therequirements, particularly with regard to synchronism, on the pumps wererelatively high, as even small deviations in the pump speeds will have acritical effect on the laminate thickness and thus enable a localescaping of light from the fluid. With the new method, however, it mustonly be observed that the fluid is led into the auxiliary fluid at theright speed.

[0008] Preferably, the auxiliary fluid is water. Water, also pure water,is available to the required extent in most laboratories. The handlingof it is simple. The refractive index of water is known or can easily bedetermined. Water is not aggressive in connection with many fluids, sothat in most cases measuring results will not be distorted.

[0009] Preferably, the fluid is composed by a carrier and a sample. Thisis particularly advantageous, when the physical properties of the sampleare similar to those of the auxiliary fluid, for example, when water isused as auxiliary fluid and the sample is sewage water from apurification plant, in which, for example, the contents of ammonium mustbe determined. By means of the carrier, the properties of the fluid cannow be changed to such an extent that they deviate sufficiently from thephysical properties of the auxiliary fluid.

[0010] It is particularly advantageous that the carrier is a fluid witha higher viscosity than that of the auxiliary fluid. For example,glycerol can be used as carrier, as it has a high viscosity. By means ofthe viscosity of the carrier, it is possible to get influence on thespeed, at which the fluid must be led through the measuring channel.

[0011] Preferably, the light is sent through the measuring channel, assoon as the fluid is supplied. Thus, some kind of reference signal isobtained. The light will practically already reach its receiver, whenonly the auxiliary fluid is in the measuring channel. As soon as thefluid is added, the signal depending on the light will inevitablychange. When changes no longer occur, it can be assumed that themeasuring distance of the measuring channel is filled with fluidsurrounded by the auxiliary fluid, and that in fact the subsequentmeasuring will take place on the predetermined length of the fluid.Thus, easily reproducible results can be obtained.

[0012] Advantageously, a U-shaped measuring channel with a base and twolegs is used, the light being led through the base. This involvesseveral advantages. Firstly, the light beam can get in and out throughboundaries of the fluid, which are directed more or less vertically tothe light beam. In this case, also with boundaries having heavilydifferent refractive indexes, the reflection of the light beam remainslow. Secondly, in this connection, at least the beginning of themeasuring can be determined with a high reliability. The measuring cannamely start, as soon as the peak of the fluid enters the base andcrosses the light beam.

[0013] Preferably, the auxiliary fluid is replenished at a predeterminedrate that depends on the supply rate of the fluid. This is particularlyadvantageous, when the fluid is available as “plugs” or blocks with alimited length. In this case, the next supply process can followimmediately after the passing of a sample. However, also with longersamples the careful supplying of the auxiliary fluid can beadvantageous, to prevent the boundary layer between the wall of themeasuring channel and the fluid from getting too thin.

[0014] The invention is useful in different measuring processes. Besidesthe absorption or transmission measurings described here, for example,also fluorescence or chemi-luminescence processes can be used.

[0015] In the following, the invention is described on the basis of apreferred embodiment in connection with the drawings, showing:

[0016]FIG. 1 a schematic view of an analysis system

[0017]FIG. 2 various stages of a measuring process

[0018]FIG. 1 shows an analysis system 1, for example in the shape of a“lab on a chip” in micro-size. Such an analysis system will then onlyrequire a base area of a few square centimetres. The consumption ofreagents and energy is extremely low, so that such analysis systems canalso for a certain period of time be operated autonomously, that is,without supplying additional energy or reagents.

[0019] The analysis system 1 has a first pump 2, supplying an auxiliaryfluid, having a low optical refractive index, from a tank 3 into ansupply channel 4 for a measuring channel 5. The measuring channel can becauterised or cut into a substrate 6, preferably of glass or plastic.For example, the measuring channel has a square cross section of 0.5mm×0.5 mm. It is U-shaped and has a base portion 7 and two leg portions8, 9, the leg portion 8 being an inlet channel and the leg portion 9being an outlet channel. The base portion 7 forms a cyvette that has alength of 30 mm. Thus, the cyvette has a volume of 7.5 micro litres. Themethod used for filling the measuring channel 5 with water isinsignificant.

[0020] A sample tank 11 (in stead of a sample tank, a sampling devicemay be provided, which takes water from a purification plant or producesa sample fluid by means of dialysis from the water of a purificationplant) is connected with a mixing device 12 that mixes the sample with acarrier from a carrier tank 13. In the present case, the carrier isglycerol, which has a higher viscosity. The carrier is decisive for thesize of the refractive index. Together, carrier and sample now form afluid, which, in the following, is also called “core fluid”. Therefractive index of the core fluid is higher than that of water.

[0021] Through a second pump 14 and a supply channel 15, the core fluidis led to the inlet channel 8 and from there into the cross section.Like the rest of the measuring channel 5, the inlet channel 8 is filledWith water.

[0022] The two pumps 2, 14 are controlled by a controlling device 10that is able to synchronise the supply rates of the two fluids 20, 21(FIG. 2a) in such a way that the desired parabolic course of the corefluid 21 in the fluid 20 occurs.

[0023] A light source 16 sends light with a certain wavelength range ora certain wave length, for example, from the UV-range or the IR-range,via a light conductor 17 into the cyvette 7. The outlet of the cyvette 7is connected with a detector 19 via an additional light conductor 18.The detector 19 measures the absorption of the core fluid and based onthis calculates the concentration of a substance in the sample, forexample the ammonium content in the sewage.

[0024]FIG. 2 is a detailed view of the conditions in the measuringchannel 5. The measuring channel 5 is filled with water 20 (FIG. 2a),and the core fluid 21 is supplied through the inlet channel 8.Simultaneously with the supply of the core fluid 21, the light source 16and the detector 19 are activated, that is, the light source 16 sendsout light extending in the longitudinal direction of the cyvette 7. Thisis marked by two arrows 22, 23. At the time shown in FIG. 2a, the lightbeam thus “measures” clean water.

[0025] In the embodiment shown, the core fluid is supplied at a speed often millilitres/hour. After approximately two seconds, the profile inFIG. 2b has been reached, and after approximately three seconds, thecore fluid 21 has also passed the second leg or the outlet channel 9 ofthe measuring channel 5.

[0026] The fact that the light source 16 and the light sensor 19 areactivated simultaneously with the supply of the core liquid 21, makes itpossible to follow a signal course, and the final measuring can be madeat the maximum of this signal, as at this stage the cyvette 7 is filledto the desired extent by the core fluid 21.

[0027]FIG. 2b shows the parabolic flow profile that has occurred due tothe flow speed of the core fluid 21. The core fluid, having a diameterW, is surrounded by a laminar water layer with a thickness B. Thethickness B is variable over the length of channel, however must exceeda minimum value, as otherwise the measuring signal will drop rapidly, asthe light in the core fluid can escape through the wall of the cyvette7. Also when the flow speed of the core fluid is too low, the thicknessB may become too small due to the diffusion of the core fluid into theauxiliary fluid. Here, the cross section of the flow profile is shown asa parabola, however, in practice it has substantially the shape of aparaboloid. In a circular channel the profile is an almost idealparaboloid. Also the water 20 can be replenished at a certain rate. Whensubstantially maintaining a predetermined relation between the speeds ofcore fluid 21 and water 20, the parabolic peak shown occurs, which, likea cone, pushes the water 20 aside to surround the core fluid 21. Thisrelation does not have to be strictly observed. The exact speed of thecore fluid 21 depends on several parameters. The larger the differencesbetween the densities of the two fluids are, the higher must the speedof the core fluid be, in order to prevent the core fluid from sinking tothe bottom of the measuring channel 5. This is particularly important insystems, in which the fluids are to flow in horizontal channels. Alsosmaller cross sections or longer cyvettes 7 require higher flow speeds.Depending on the dimensions and the fluids chosen, there is an upperlimit of the flow speed, at which turbulences occur and the laminarstructure collapses. To prevent this from happening, the Reynolds figureshould be lower than 2000.

[0028] The core fluid 21 can be a “plug” or a block with a limitedlength. In this case, the passing of each block will cause the system torevert to the original state, that is, the measuring channel 5 is filledwith water 20 and is then ready to adopt the next sample. Due to thewater layer between the sample and the measuring channel 5, a pollutionof the measuring channel has not occurred.

[0029] However, the sample can also be available as a continuous, thatis, longer block. Also in this case, merely the flow speed of the corefluid 21 must be set so that laminar conditions are maintained. In thiscase, a measuring can, for example, be performed over a longer period.

[0030] Finally, it is also possible, to supply laminated sections of thecore fluid 21, in stead of such continuously extending sections, intothe measuring channel 5. The principle of laminating such samples isknown from, for example, DE 44 11 266 A1. Here, a particular opportunityof analysing appears. For example, the light can be led specificallythrough the “peaks” of the parabolic profiles to obtain an optical lenseffect.

1. Method for the optical analysis of a fluid, in which the fluid thathas a first refractive index, is surrounded by an auxiliary fluid with asecond refractive index, the first refractive index being larger thanthe second refractive index and light being sent through the fluid,characterised in that firstly the auxiliary fluid is led into ameasuring channel, until it is filled, and secondly the fluid is ledinto the auxiliary fluid within the cross section with a flow speed,which results in a parabolic flow profile.
 2. Method according to claim1, characterised in that the auxiliary fluid is water.
 3. Methodaccording to claim 1 or 2, characterised in that the fluid is composedby a carrier and a sample.
 4. Method according to claim 3, characterisedin that the carrier is a fluid with a higher viscosity than that of theauxiliary fluid.
 5. Method according to one of the claims 1 to 4,characterised in that the light is sent through the measuring channel,as soon as the fluid is supplied.
 6. Method according to claim 5,characterised in that a U-shaped measuring channel with a base portionand two leg portionss is used, the light being led through the baseportion.
 7. Method according to one of the claims 1 to 6, characterisedin that the auxiliary fluid is replenished at a predetermined rate thatdepends on the supply rate of the fluid.